A battery is a device that converts chemical energy into electricity. A well-known example of a primary cell is standard alkaline battery. The electrolyte is an alkaline gel, usually potassium hydroxide. The positive electrode is made of magnesium dioxide; the negative electrode is made of zinc. Well-known rechargeable batteries are nickel-cadmium, nickel-metal hydride, or lithium ion batteries. Secondary batteries self-discharge more rapidly than primary batteries and must be recharged before they are used.
Manufacturers of battery-powered, portable consumer electronics typically must select one form factor, or type of battery to use-typically either AA-sized batteries or a rechargeable pack. Most portable electronic devices that can accept AA-sized batteries can accept a variety of battery chemistries, both rechargeable and non-rechargeable. The five types of batteries available to consumers in the AA form factor are alkaline; high-drain alkaline; high-energy lithium; nickel-metal hydride; and nickel cadmium. These different battery chemistries do not behave the same way during discharge. This difference in discharge poses a problem for fuel gauging. Additionally, if one wishes to recharge the batteries in the device, the batteries need to be identified as rechargeable so that non-rechargeable batteries are not subject to accidental charging.
Conventional portable electronic devices use one of two charge monitoring techniques:
use only an application specific battery pack, calibrating the fuel gauges and charging accordingly; or PA1 use standard AA batteries, calibrate the fuel gauge for a predetermined chemistry (e.g., alkaline) and only allow charging outside the device.
These conventional approaches have obvious disadvantages. In the first technique, the user is restricted to using very specific, often expensive batteries and the device may be unusable while the batteries are charged. The second technique suffers the disadvantage of fuel gauging being inaccurate for any battery chemistry other the predetermined chemistry.
Typically, battery fuel gauging in portable electronic devices has been restricted to one of two methods: (1) monitoring change in voltage over change in time (dv/dt) and (2) monitoring power exiting and entering the batteries. The first method, dv/dt, is only accurate with alkaline batteries since they have a predictable sloping discharge voltage profile. Most non-alkaline batteries have a much flatter voltage discharge profile such that it is difficult to see any significant change in voltage until the voltage is almost completely discharged. Thus, if the device is configured to gauge battery fuel using the first method, dv/dt, the voltage will read close to 100% until the batteries are approximately 90% discharged.
The second method is generally used for non-alkaline batteries, i.e., batteries with a relatively flat voltage discharge profile. A limitation of this method is that it requires prior knowledge of how much approximate power can be drained from the battery before it is fully discharged. Thus, this method is typically used in situations where a known battery chemistry will be used repeatedly. If the device could determine chemistry, then this method of monitoring power exiting and entering the battery could be used more readily.
Since alkaline batteries are by far the most common battery type, most portable electronic devices use fuel gages that monitor dv/dt. With the proliferation of alternative battery chemistries and the attendant reduction in cost, consumers have greater latitude is selecting their batteries. These dv/dt fuel gauges will fail should the consumer insert a high-energy lithium or nickel-metal hydride cell in the device, for example, showing an inaccurate level of fuel available.
There is a need in the industry to ensure consumers of portable electronic devices can rely on the battery fuel gauges that are provided with the device. Additionally, there is a need to provide safe in-device battery recharging in devices that accept both rechargeable and non-rechargeable batteries.